datetime xtype validation issue

I am using a datetime widget to capture expiration date(custom project level property) for a asset

I want to validate the date entered using the widget, to make sure date + time is not in the past (We would let user to enter today as expiration date, provided the time is in the future).

I am able to call the call the validation functions separately for date and time i.e

dateConfig {validator: function(value){return isAFutureDate(value);}} //here value is only date

timeConfig {validator: function(value){return isAFutureDate(value);}} //here value is only time

I tried calling a custom function on change event, but i am seeing-  this is not getting called. I tried something as simple as change: function(this,n,o){alert("my value was changed");}

But i am not getting any alert messages

How can i call a call a validation function easily, with the value of both of time and date sent to it

Thanks in Advance

CQ 5.5